Overhaul the plugin cache file with a new msgpack+brotli format (#12579)

# Description

- Plugin signatures are now saved to `plugin.msgpackz`, which is
brotli-compressed MessagePack.
- The file is updated incrementally, rather than writing all plugin
commands in the engine every time.
- The file always contains the result of the `Signature` call to the
plugin, even if commands were removed.
- Invalid data for a particular plugin just causes an error to be
reported, but the rest of the plugins can still be parsed

# User-Facing Changes

- The plugin file has a different filename, and it's not a nushell
script.
- The default `plugin.nu` file will be automatically migrated the first
time, but not other plugin config files.
- We don't currently provide any utilities that could help edit this
file, beyond `plugin add` and `plugin rm`
  - `from msgpackz`, `to msgpackz` could also help
- New commands: `plugin add`, `plugin rm`

# Tests + Formatting

Tests added for the format and for the invalid handling.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

- [ ] Check for documentation changes
- [ ] Definitely needs release notes

crates/nu-plugin/src/lib.rs

@@ -78,10 +78,10 @@ pub use serializers::{json::JsonSerializer, msgpack::MsgPackSerializer};
 // Used by other nu crates.
 #[doc(hidden)]
 pub use plugin::{
-    create_plugin_signature, get_signature, serve_plugin_io, EngineInterfaceManager, GetPlugin,
-    Interface, InterfaceManager, PersistentPlugin, PluginDeclaration,
-    PluginExecutionCommandContext, PluginExecutionContext, PluginInterface, PluginInterfaceManager,
-    PluginSource, ServePluginError,
+    create_plugin_signature, get_signature, load_plugin_cache_item, load_plugin_file,
+    serve_plugin_io, EngineInterfaceManager, GetPlugin, Interface, InterfaceManager,
+    PersistentPlugin, PluginDeclaration, PluginExecutionCommandContext, PluginExecutionContext,
+    PluginInterface, PluginInterfaceManager, PluginSource, ServePluginError,
 };
 #[doc(hidden)]
 pub use protocol::{PluginCustomValue, PluginInput, PluginOutput};